Wireless communication called “millimeter wave” communication can realize higher communication speed by using a high-frequency electromagnetic wave. Examples of the main applications of millimeter-wave communication include wireless access communication for a short distance, an image transmission system, simplified wireless communication, and automobile collision prevention radars. Furthermore, at present, technology development for millimeter-wave communication, which is directed toward use promotion, such as realization of large capacity and long distance transmission, size reduction of wireless apparatuses, and reduced cost, has been performed. Here, the wavelength of a millimeter wave corresponds to 10 mm to 1 mm, and the frequency corresponds to 30 GHz to 300 GHz. For example, in wireless communication using a 60 GHz band, since channel assignment is possible in GHz units, very high-speed data communication can be performed.
A millimeter wave has a shorter wavelength and a stronger property of rectilinear propagation compared to microwaves that have become widely popular in a wireless LAN (Local Area Network) technology or the like, and can transmit a very large amount of information. On the other hand, since the attenuation of a millimeter wave as resulting from reflection is intense, for a wireless path for performing communication, a direct wave, and a wave reflected approximately one time at most are mainly used. Furthermore, since the propagation loss of a millimeter wave is large, a millimeter wave has a property such that a radio signal does not reach far places.
In order to compensate for such a travel distance problem of a millimeter wave, a method is considered in which an antenna of a transmitter/receiver is made to have directivity, a transmission beam and a reception beam thereof are directed in a direction in which a communication party is positioned, and a communication distance is extended. The directivity of a beam can be controlled by, for example, providing each of transmitters/receivers with a plurality of antennas, and by changing the transmission weight or the reception weight for each antenna. In millimeter waves, since reflected waves are hardly used, and a direct wave is important, beam shaped directivity is suitable, and a sharp beam is used for directivity. Then, after the optimum directivity of the antenna is learned, millimeter-wave wireless communication may be performed.
For example, a wireless transmission system has been proposed in which second communication means using communication of any one of electrical power line communication, optical communication, and sound wave communication transmits a signal for determining the directional direction of a transmission antenna, and the direction of the transmission antenna is determined, and thereafter, first communication means performs wireless transmission among transmitters/receivers using a radio wave of 10 GHz or higher (see, for example, Patent Document 1).
Furthermore, a method of extending a communication distance by using the directivity of an antenna has been used in IEEE 802.15.3c, which is a standard specification of wireless PAN (mmWPAN: millimeter-wave Wireless Personal Area Network) using a millimeter-wave band.
By the way, in wireless communication, it is known that a hidden terminal problem such that an area in which communication stations cannot directly communicate with one another exists occurs. Since negotiation cannot be made among hidden terminals, there is a probability that transmission operations will collide with one another. As a methodology for solving a hidden terminal problem, a “virtual carrier sense” can be given. According to the virtual carrier sense, the hidden terminal predicts a period in which the medium is used, and stops a transmission operation without performing physical carrier sense during the relevant period. Specifically, duration information for reserving a medium has been described in the header of a MAC (Media Access Control) frame for requesting a stop of transmission. A peripheral station receiving a frame destined for another station expects that a medium is used in the period corresponding to the duration information, and sets a network allocation vector (NAV).
A representative example of a signal transmission/reception sequence using virtual carrier sense is RTS/CTS handshake, and is widely used in a wireless LAN system, such as IEEE 802.11. The communication station of the data transmission source transmits a transmission start request frame RTS (Request To Send), and starts the transmission of data frames in response to the reception of an acknowledgement frame CTS (Clear To Send) from the communication station of the data transmission destination.
Here, each of the control frames of RTS and CTS has a meaning of confirming the preparation situation for data transmission among transmitters/receivers and making hidden terminals in the surroundings not obstruct data transmission. When a hidden terminal for the data transmission side (RTS transmission station) receives a CTS destined for another station, the hidden terminal sets a transmission stop period on the basis of the duration information described in the MAC header thereof. Consequently, it is possible for a data receiving side (CTS transmission station) to avoid a collision with a transmission frame by the relevant hidden terminal, and can reliably receive the data frame. Furthermore, a hidden terminal for the data receiving side (CTS transmission station) receives an RTS destined for another station and sets a transmission stop period.
In the wireless PAN standard IEEE 802.15.3c (described above) using a millimeter-wave band, also, a collision avoidance procedure using an RTS/CTS handshake has been adopted. For example, beamforming of a transmission/reception beam is used with regard to data frames only, and control frames, such as RTS, CTS, and ACK, are transmitted as omni-directional frames. Then, in the surroundings of the communication apparatus, a signal transmission/reception procedure using RTS/CTS handshake is performed, and when the communication apparatus receives an RTS or a CTS that is not destined for its own station, the communication apparatus needs to set a transmission stop period.
However, in the case of a millimeter-wave communication apparatus using beamforming of a transmission beam in the manner described above, despite that the communication apparatus exists in a range in which an RTS or a CTS can reach, even if the communication apparatus transmits a beamformed frame (beamformed packet) within a transmission stop period, signal transmission/reception procedure in an RTS transmission station or in a CTS transmission station is not sometimes obstructed depending on the direction (or the direction of the position of the communication party) in which the transmission beam id directed.
For example, it is assumed in the directional communication system shown in FIG. 11 that, after RTS/CTS handshake is performed between an STA_A and an STA_B, the STA_A is transmitting a data frame by directing a transmission beam in the direction of the STA_B. On the other hand, an STA_C in the surroundings of the STA_B is assumed to want to transmit frames to the STA_D. At this time, if the STA_C transmits frames by directing a transmission beam in the direction of the STA_D, the STA_C does not obstruct the frame reception of the STA_B. However, when the STA_C receives a CTS from the STA_B, the STA_C sets a transmission stop period, and thus withholds the operation of transmitting frames to the STA_D.
In other words, even if the millimeter-wave communication apparatus receives an RTS or a CTS, which is not destined for its own station, the millimeter-wave communication apparatus does not need to stop the frame transmission operation as long as beamforming is applied. If the frame transmission operation is stopped unnecessarily in spite of that, the number of communication stations that can be communicated with at the same time in the system is uselessly reduced, and there is a concern that the throughput of the entire system is decreased.